4 ms·

Though I am curious about why he didn’t switch to using predicated instructions for the tail loop. I’ve switched to that pattern when writing AVX512.

• moonchild 4y ago
  'Tail handling' in general is an annoying aspect of simd. Masks are great, but no panacea--in particular, if you unroll, then you cannot take care of the tail with a single masked instruction. There are various solutions to this. I favour overlapping accesses, where that's feasible (following a great deal of evangelism from mateusz guzik); a colleague uses a variant of duff's device; you can also just generate multiple masks. I would expect the linked code is just intended as a quick poc, so it does not bother to be optimal.
  • renox 4y ago
    > 'Tail handling' in general is an annoying aspect of simd. Only for "classical" SIMD, ARM SVE2 show a way to solve this issue cleanly. Not sure where you can use SVE2 though..
    • deleted 4y ago
      [deleted]
    • kristianp 4y ago
      Not SVE2, but lemire has a post about using SVE on Amazon Graviton 2 processors: https://lemire.me/blog/2022/07/14/filtering-numbers-faster-with-sve-on-amazon-graviton-3-processors/ https://lemire.me/blog/2022/07/14/filtering-numbers-faster-w...
      • kristianp 4y ago
        Correction, Graviton 3.
  • janwas 4y ago
    :) For anyone interested, here's a brief discussion of tail handling: https://github.com/google/highway#strip-mining-loops https://github.com/google/highway#strip-mining-loops (Overlapping is indeed cool where it works - idempotent operations.)
    • moonchild 4y ago
      > idempotent Roughly speaking, while you do need idempotence for reductions, you do not need it for maps. (Of course, plenty of things don't look quite like reductions or maps--or they look like both at once--and each is unique.)